The present invention concerns a bipolar transistor having an SiGe alloy base layer and, more in particular, it relates to a bipolar transistor suitable to a ultra-high-speed digital IC and microwave or millimeter wave wireless communication IC, as well as a communication system in which the bipolar transistor is applied.
Prior art related to a bipolar transistor using B-doped SiGe alloy (SiGe alloy) for a base is described in Technical Digest of International Electron Devices Meeting (IEDM), 1997 (pages 791-794). The SiGe base bipolar transistor of the prior art is to be explained with reference to FIG. 17(a) and FIG. 17(b). Graphs in the drawing show typical two examples of a depth profile of impurity concentrations and Ge contents in an active region of a SiGe base bipolar transistor of the prior art (refer to FIG. 4 showing a cross section for the main portion of a transistor; along broken line A) of the prior art. In the graphs, a region of a shallow depth is for an emitter and a portion with a large depth is for a collector. In the example shown in FIG. 17(a), the Ge content increases at a constant grade toward the collector in a region of a base layer in which B is doped, reaches a maximum value at an edge of the base layer, and is kept at the maximum value in a region of a predetermined width in a base-collector depletion layer. In the example shown by FIG. 17(b), the Ge content has a constant value in a region of the base layer in which B is doped and in a region of a predetermined width in the base-collector depletion layer.
For improving the operation speed of a bipolar transistor, it is necessary to shorten a base transit time of carriers and lower a base resistance. For this purpose, it is necessary for the depth profile of B in the base layer to make the width as narrow as possible and increase the concentration as high as possible. However, in the prior art described above, since B in the base layer is diffused by a heat treatment after forming the base layer, it results in a problem that the depth profile becomes broad to widen the width. In addition, since the broadening for the width of the profile due to diffusion increases as the concentration of B is higher, it was difficult to make the restriction for the width of profile and increase of the concentration compatible with each other.
Another problem in the prior art transistor is that an effective base width is widened to lower the operation speed if the collector current is increased. This is caused by injection of holes for neutralizing the negative charges due to increase of a collector current density in a region at low impurity concentration, so that an energy band structure for a base-collector junction is changed as shown in FIG. 18.
FIG. 15 shows dependence of a diffusion coefficient of B in an SiGe alloy on a Ge content. As the Ge content increases, the B diffusion coefficient decreases to lower the diffusion speed. The problem of increasing the depth of B can be solved by utilizing this phenomena by adopting the following means.
If the Ge content is increased in a region adjacent to the B-doped layer of the base, that is, in an emitter-base depletion region or a base-collector depletion region, since the diffusion speed of B in the portion is lowered, widening of the width in the depth profile of B can be suppressed.
When taking notice only on the decrease of B diffusion, the total Ge contents may be increased in the emitter-base junction depletion region, the base layer, and the base-collector depletion region, but the total amount of Ge in the SiGe alloy layer is excessively increased. Therefore, this causes strong stresses in the SiGe alloy layer due to the difference of covalent bonding radius between Si and Ge, to cause a side-effect of forming crystal defects.
The side effect can be suppressed by controlling the Ge content higher in a portion adjacent to the B-doped layer of the base and lower in the B-doped layer as much as possible. Since the total Ge content can be decreased by this structure, occurrence of crystal defects can be suppressed. In this case, broadening of the distribution of B caused by the reduction of the Ge content in the B-doped layer is almost negligible. This is because the distribution of B is generally uniform in the B-doped layer, so that B is less diffused and the degree of the diffusion coefficient in this region gives less effect on the broadening of the depth profile of B. For lowering the Ge content in the B-doped layer and kept it high in the portion adjacent therewith thereby decreasing the total Ge content as low as possible, the Ge concentration may be changed abruptly as much as possible at both ends of the B doped layer.
Further, the effect of suppressing the broadening of the B profile compared with the prior art can be attained also by making the Ge content in the emitter-base junction at least equal with the content in the B-doped layer though it is not higher than the content in the B-doped layer, by the same reason as described above.
Further, the problem that the effective base width is increased by the increase of the collector current can also be improved by the method described above of changing the Ge concentration as sharp as possible at the edge of the B-doped layer on the side of the collector. This reason is to be explained below. In a transistor of the prior art, an effective base width is increased as the collector current density increases as shown in FIG. 18 since the grade of the band become less steep at the edge of the base on the side of the collector. On the other hand, when the Ge concentration is changed abruptly at the edge of the B-doped layer, a nodge is formed in a valance band at a position at which the Ge concentration changes as shown in FIG. 2. The reverse V-shaped nodge on the side of the collector has a function of hindering the effect of smoothing the grade of the band in that portion when the collector current density increases. This is because a great amount of holes are accumulated at that portion if the grade is reduced and electrical neutral can no more be kept in the vicinity thereof. The effect of suppressing the change of the gradient of the band with increase of the collector current density can improve the problem of lowering the operation speed in a case of increasing the collector current.
Based on the considerations as described above, the present inventor provides a bipolar transistor of the following structure and a communication system by applying such a bipolar transistor.
At first, a bipolar transistor using a B-doped Si and Ge alloy (SiGe) according to the present inventor has a basic feature in that the maximum value of Ge content in an emitter-base junction depletion region and a base-collector junction depletion region is greater than an average value in the base layer. In this bipolar transistor, it is preferred that the grade of Ge content in a region in which the Ge content is increased from the vicinity of the edge of the base on the side of a collector to the collector is made greater than the average grade of Ge content in the base layer.
Further, another feature of the present invention resides in a bipolar transistor using a B-doped SiGe alloy (SiGe) for a base in which the maximum value of the Ge content in the base-collector junction depletion region is set greater than the average value in the base layer, wherein the grade of Ge content in the region in which the Ge content is increased from the vicinity of the edge of the base on the side of a collector to the collector is made greater than the average grade of the Ge content in the base layer.
In a further aspect of a bipolar transistor according to the present invention, a bipolar transistor using a B-doped Si and Ge alloy (SiGe) for a base in which the maximum value for the Ge content in the base-collector depletion region is set greater than an average value in the base layer has a region in which the Ge content is constant from the edge of the base layer on the side of the emitter to the emitter-base junction. In this structure, it is preferred that the grade of Ge content in the region in which the content increases from the vicinity of the edge of the base layer to the collector is made greater than the average grade of Ge content in the base layer.
Further, in an optical transmission system according to the present invention comprising
an optical receiver system having an a photodetector for receiving an optical signal and outputting an electric signal, a first amplifier for receiving the electric signal from the photodetector, a second amplifier for receiving the output from the first amplifier, a decision circuit for converting the output from the second amplifier into a digital signal in synchronization with a predetermined clock signal, and a circuit for separating and converging the digital signal, and
an optical transmitter system having a circuit for synthesizing multiple digital signals, a semiconductor laser and a semiconductor laser driver for driving the laser,
at least one of transistors in the first and the second amplifiers, the decision circuit, a multiplexer for digital signals, a multiplexer for multiple digital signals and the semiconductor laser driver is constituted with the SiGe base bipolar transistor as defined above in either one of the optical receiver system and the optical transmitter system.
Further, in a millimeter wave transmission system according to the present invention having a receiving antenna for millimeter wave (frequency: 30 GHz-300 GHz), a first amplifier for amplifying a received electric signal from the antenna, a receiving mixer for receiving the output from the first amplifier and stepping-down the frequency, a first oscillator, a transmitting mixer for receiving a transmission electric signal and stepping-up the frequency, a second oscillator, and a second amplifier for receiving the output of the transmission mixer and amplifying the power,
at least one of the transistors in the first and the second amplifiers, the first and the second oscillators and the transmission mixer is constituted with the SiGe base bipolar transistor as defined above.